An ion source is a device producing a beam of charged particles (heavier than electrons) suitable for transport to an experimental setup or to an application, such as accelerator injection, ion implantation, fusion driving, or ion propulsion. The critical element is formation of a beam, rather than simply plasma generation. The ion beam may be used for various purposes in thin film technologies, including but not limited to cleaning substrates, surface activation, polishing, etching, direct deposition of thin films, and ion beam sputter depositions utilizing various targets.
Though different types of ion sources exist, thin film technologies most commonly utilize ion sources with grids or closed electron drift.
Closed electron drift sources consist of three subgroups: 1. stationary plasma thrusters (SPT); 2. plasma accelerators with closed electron drift and a narrow acceleration zone (anode layer thrusters); and 3. end hall ion sources. (see e.g. U.S. Pat. No. 4,862,032, filed on Oct. 20, 1986).
Since their discovery, plasma accelerators with a closed electron drift and a narrow acceleration zone have been the basis for a wide variety of ion sources, named anode layer accelerators.
Devices utilizing plasma accelerators with closed electron drift and a narrow acceleration zone can be used for thin film technology and plasma chemistry. These sources are capable of generating ion beams with different configurations, shaped, for example, as rings and ellipses. They can be used for ion treatment of metal and nonmetal targets, as well as cleaning, etching and activation of surfaces. In addition, they can process materials without an additional electron emitter, although in the case of nonconductive and dielectric targets, under compensation of the ion beam by electrons results in a positive charge at the surface. The positive charge at the surface repels the incoming ion beam and thus reduces the efficiency of ion treatment.
FIG. 4 is a cross sectional view of a plasma accelerator having a closed electron drift and a narrow acceleration zone, the ion source 400 comprises a magneto-conductive housing 402 acting as a cold cathode. The ion source contains a circumferentially closed discharge channel 404, for ionization and acceleration of the operational gas. The discharge channel is formed by the inner walls of the magneto-conductive housing 402 and the circumferentially closed anode 406, which is placed coaxially inside the magneto conductive housing 402 and positioned along the discharge channel 404 for the formation of the plasma discharge space. The ion source 400 also contains means for the establishment of a magnetic field 408 in the azimuthally-closed channel (discharge channel). The discharge channel and anode are arranged within the magnetically conductive housing symmetrically with respect to the ion-emitting slit/aperture 410. The ion source emits an ion beam 412, through the ion-emitting slit/aperture 410. The emitted ion beam may be directed onto a substrate 414.
The strong magnetic field traps electrons in the discharge channel 404, but the electrons oscillate and drift in the direction perpendicular to the E×B plane in the presence of magnetic (B) and electric field (E). In other words, the electrons are induced to drift circumferentially in the discharge channel 404. Drifting electrons repeatedly collide with the operational gas atoms delivered into the discharge channel 404, thus creating ion flux that is accelerated outward through the ion-emitting slit/aperture 410 of the discharge channel 404 due to the strong electrical field between anode 406 and cathode 402.
The size of these ion sources can be easily scaled from centimeters to meters in length and configured in various emission shapes. Due to their simplicity and robustness these ion sources, as described above, have become popular for large area web and glass treatment. However, there are problems with the current design of these ion sources, which prevent their wide acceptance for use in thin film technology. During treatment of the dielectric substrates these sources may produce magnetron style discharge outside of the source (frequently this discharge is explained as a diffuse mode of operation of the ion source). This same effect may occur at higher operational pressures. This discharge will sputter cathode material and contaminate the treated articles. As discussed above, the main reasons for the creation of the magnetron discharge and subsequent erosion of the pole pieces of the cathode and contamination of the treated articles (substrates) is a higher process pressure and/or uncompensated charge of the substrates. This phenomena is explained in the paper “Autocompensation of an ion beam in an accelerator with an anode sheath” Bizyukov, A at.el Published in Technical Physics Letters, Volume 23, Number 5, May 1997, pp. 403-404(2). Publisher: MAIK Nauka Interperiodical
For many applications, the above described contamination is unacceptable. Thus, there are a number of designs directed towards reducing contamination of the treated parts.
For example U.S. Patent Publication No 20050040031, published Aug. 16, 2004, describes reducing the amount of substrate contamination by using a shield configuration that blocks the contaminants from impinging the substrate after the substrate passes through the etching beam while the substrate is moving in front of the ion source. This approach will reduce but not eliminate contamination resulting from a dynamic mode, i.e. the substrate moving in relation to the ion source. This approach is not applicable for the process in a static mode, i.e. when the substrate and ion source are not moving relative to each other.
Another example is U.S. Pat. No. 6,664,739, filed on Jun. 22, 2002, which describes reducing erosion of the component parts of the ion source. This is achieved by coating the cathode surfaces with material that, during operation of the source, will form a material that will increase electron emission. This invention claims only to reduce contamination but not to eliminate it to the point where it is possible to work with all substrates.
There are additional inventions that attempt to reduce contamination and increase ion beam effectiveness by optimizing the magnetic field of the ion source. U.S. Pat. No. 6,919,672 filed on Apr. 10, 2003 and U.S. Pat. No. 6,864,486 dated Mar. 8, 2005 are some of the examples of this approach. These inventions are, in general, related to the optimization of the magnetic field and the pole pieces of the ion source.
FIGS. 5 and 6 show top down views of differently shaped ion sources. FIG. 5 depicts a circular ion source and FIG. 6 depicts an elliptical ion source. The ion source has slits 502 and 602 respectively, through which the ion beam exits the ion source.
The current invention is an improvement of ion sources based on the plasma accelerator with closed electron drift and a narrow acceleration zone, also commonly referred as Anode Layer Ion Sources.